In the past ten years, a number of proceedings of symposia on the structure and function of proteolytic enzymes have been published. Their coverage of acid proteases has been limited, mainly due to the lack of significant new information on the structure of these enzymes. In the last four years, however, the primary and tertiary structures of a number of acid proteases have been determined, prompting the need to discuss the meanings of the old data and the possibilities for new experimentations. It was for this purpose that this book was organized. It took place at the University of Oklahoma on November 21-24, 1976. This book is a collection of the main lectures delivered at the Conference. Acid Proteases, by definition refers to a group of proteases having an optimal pH in acidic solutions. The classic examples are pepsin and chymosin. Some catalytic features are obviously shared by these proteases, most notably, their inhibition by pepstatin. The use of active center-directed inactivators such as diazoacetylnorleucine methyl ester and 1,2-epoxy-3-(p-nitrophenoxy)propane has shown that two catalytic aspartyl residues are present in most of these enzymes. These apparent common features have prompted the suggestion by several investigators to name this group of enzymes "aspartyl proteases" or "carboxyl proteases". Such proposals are particularly valid if one considers that the optimal pH of renin is about 6, but its catalytic residues and mechanism obviously belong to that of the acid proteases. Regardless of the name eventually adopted, there is little question that this is a group of proteases with a structure-function relationship different from other groups of proteases. They appear to have some important functions in various biological systems.